Efficiency, Effectiveness, and Surfactant Structure

Because the extent of reduction of the surface tension of a solution depends on the substitution of surfactant for solvent molecules at the interface, the relative concentration of surfactant in the bulk and interfacial phases should serve as an indicator of the adsorption efficiency of a given surfactant and, therefore, as a quantitative measure of the activity of the material at the solution-vapor interface. For a given homologous series of straight-chain surfactants in water, CH3(CH2) —S, where S is the hydrophilic head group and n is the number of methylene units in the chain, an analysis based on the thermodynamics of transfer of a surfactant molecule from the bulk phase to the interface leads to the conclusion that the above-defined efficiency of adsorption is directly related to the length of the hydrophobic chain. The efficiency can be defined mathematically by the expression 

If a surfactant possesses two polar groups, the methylene groups lying between the two polar groups contribute an effect equivalent to approximately one-half that found for such groups located in the main body of the hydrophobe. 

In cationic surfactants, the presence of short-chain alkyl groups (fewer than four carbon atoms) attached to the nitrogen seem to have little effect on the efficiency of adsorption of the molecule. The dominant factor will always be the length of the primary hydrophobic chain. That effect is true regardless of whether the alkyl groups are attached to a quaternary ammonium group, an amine oxide, or a heterocyclic nucleus such as pyridine. 

Polyoxyethylene (POE) nonionic surfactants with the same hydrophobic group and an average of 7-30 OE units, exhibit adsorption efficiencies that follow an approximately linear relationship of the form 

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